The invention relates generally to a process for the treatment of textile webs and more particularly to a continuous process and apparatus for treating a textile web by the wet-in-wet application of two interacting treatment baths.
The invention is generally directed to the problems which have long existed in continuous dyeing process with direct dyes (also referred to in German as Substantiv-Farbstoffen). For example, see Fischer-Bobsien, "International Lexikon Textilveredlung+Grenzgebiete", 4th edition 1975, columns 419-422. The dye bath of such a process is applied in a foulard applicator with the textile web being passed through the relatively large quantity of dye liquid contained in the trough of the foulard. One of the problems with this process is the end waste runs that inevitably occur due to nonuniform treatment of the web both at the beginning and at the end of the textile web. As the first portion of the web passes through the bath, absorption by the textile web causes a corresponding depletion of the bath, which must be readjusted. It takes some time before an equilibrium is established and the concentration of the bath remains constant. Until this state is reached, the web is still running through the bath and an end waste run of typically 50 to 150 m has taken place. An end waste run is a length of fabric of different and in particular nonuniform color that cannot be used together with the rest of the fabric. Therefore, end waste runs are sold as inferior qualities or are black overdyes. In any event, end waste runs result in some loss that heretofore has not been avoidable.
Another problem with this process is that the direct dyes are not completely absorbed and the dye that is not fixed on the textile web is washed out after the steaming process. The dye yield, i.e., the ratio of the amount of dye actually fixed on the fabric to the amount of dye originally applied, is far from 100% (ideal), but rather is on order of about 60%. The unabsorbed 40% of the dye is washed "down the drain" and constitutes not only a quite considerable cost factor, but also a difficult environmental problem. The environmental problem can be severe because molecules of direct dyes contain complex-bound heavy metal ions, in particular copper ions, which are regarded as a dangerous sewage poison and are the subject of strict government regulations that must be obeyed.
It is known that the absorption of direct dyes on a fabric web generally can be improved by addition of salts, such as common salt or Glauber's salt (sodium sulfate). In this manner, the proportion of dye actually absorbed by the fibers can be increased and thus the dye loss and the pollution is reduced. However, in the continuous dyeing process, the addition of salt to the dye bath in the foulard has been found to lead to an intensification of the end waste run problem. The salt increases the absorptive capacity of the fabric such that the web rapidly takes up dye from the bath and the bath exhibits, at the beginning and toward the end of the web, still stronger concentration variations that are difficult to control. Thus, it has been thought that the addition of salt, proven per se as a means for improving the dye yield and the pollution, cannot readily be employed in continuous dyeing processes.
The invention is specifically directed to the problem of providing a continuous dyeing process for treating a textile web by the wet-in-wet application of two treatment baths such that the interaction of the two treatment baths occurs in a controlled manner, no variations in treatment result from changes in concentration of the second treatment bath and a uniform treatment over the length of the textile web is achieved.